The Scorpions have a tolerably straight, narrow, and simple tube, complicated by several pairs of straight sacs, which come off' at right angles to its anterior part, and are probably gastric crops. Below these, two bifurcating tubes, of great length and small diameter, open into a constriction of the canal. They are regarded as hepatic.
In the order of .3 follu,sea, many of whom in habit the sea, we rnay again trace a gradual ad vance of development in the intestinal canal.
The Tunicata is its lowest subdivision. Here a simple canal begins by a wide ceso phagus, that leads from the bottom of the branchial sac to a stomach or dilatation. This is surrounded by a number of hepatic follicles, that open into its intestinal end ; and it leads to a wide recurved intestine, which ends by an aperture on the upper and outer surface of the animal. Sometimes the liver varies from this description in the fact that its follicles are agg,regated.
The Brachiopoda possess a digestive ap paratus of nearly equal simplicity with the preceding. Dental structures are wanting ; and the liver is still follicular.
The Lamellibranchiata exhibit a somewhat similar condition. Their gastric cavity is sometimes preceded by a short cesophagus. From hence a comparatively simple intestine continues, with a few convolutions, through a mass of liver, to terminate, by a long straight portion, in the anus. The latter segment, or rectuni, lies along the hinge of the;r shell, and often perforates the heart in its course. Al though the liver is large and aggregate, it opens by several ducts into the gastric dila tation.* The Ga steropod a have a head, jaws, and salivary follicles. Their longer cesophagus sometimes dilates into an ingluvies or crop. Their stomach often possesses a thickened lining, and a masticatory apparatus of teeth or plates, which make it a kind of gizzard. Sometimes it is divided into two or more compartments. The large liver opens into the pyloric extremity of the stomach, or the com mencement of the intestine, by one or more ducts; or, rarely, it empties itself into the cesophagus. One or two large glandular cceca also open into the beginning of the intestine, and are regarded as a rudimentary pancreas.
The remainder of the tube is siraple, and ends anteriorly in the body, in accordance with the general structure of the animal. In the numerous herbivorous species, the intes tine is longer and more tortuous ; while the crop, the gizzard, and the masticatory appa ratus all reach a high development.
The intestinal canal of the Pteropoda is very similar. It possesses jaws and salivary glands, together with an cesophagus, a crop, a gizzard, a short and simple intestine, and a conglo merate liver that often opens by a single duct.
The Cephalopoda exhibit a marked advance of development. Their mandibles form a powerful organ of mastication ; and, in many species, salivary glands co-exist. The mouth leads to a long and dilatable cesophagus, hich descending, sometimes expands into a crop before it finally reaches the gizzard or muscular stomach. This organ is of tolerably uniform appearance. Its shape is round, or somewhat elongated ; it has a thick and whitish epithelial lining, and its muscular layer consists of two sets of fibres, each of which radiates from a central tendon on one side of the organ. The cardiac and pylo ric orifices are situated at its upper part. The intestine coming from the latter soon communicates with another cavity, which is sometimes reg,arded as a stomach. This is, in the lower Cephalopods, nearly spherical, But in many of the higher or Dibranchiate division, it is of less simple form, being triangular, elongated, or folded spirally like a snail shell. Its mucous membrane is rugous and follicular ; and the large liver, which is still supplied by arterial blood, opens into it by a single duct. The intestine continues hence as an uniform tube, which, after one or two slight curves, bends upwards to open at the base of the funnel. In some species we also find ccal appendages, the ducts of which join those of the liver before they enter the intestinal cavity. These have been supposed to constitute a rudimentary pancreas.* The alimentary canal of Fishes is simple, wide, and short, compared with that of other Vcrtebrata. Its chief subdivisions are an cesophagus, a stomach, and an intestine.